1. Field of the Invention
The present invention relates to a mobile communication system and, more particularly, to a method of controlling the timing for an uplink synchronous transmission scheme.
2. Background of the Related Art
The uplink synchronous transmission scheme (USTS) is considered an additional technology in the 3rd Generation Partner Project (3GPP). USTS eliminates intra-cell interference by transmitting data, having the same scrambling codes and different orthogonal codes, from user equipment (UE) (i.e., mobile station) to a base station (Node B). The data transmitted by the UEs are received at the same time through the control of the base station. That is, the base station transmits each UE the time alignment bit (TAB) information to align the data arrival time and each UE controls its transmit timing based on the TAB information.
The timing control for USTS goes through an initial synchronization and a tracking process. A brief timing synchronization is obtained from the initial synchronization and the timing obtained from the initial synchronization is maintained in the tracking process. Generally, the initial synchronization is performed during a call set-up process, but sometimes it is omitted and the tracking process is performed. In the later case, the initial synchronization is performed by exchanging other control information, instead of the USTS TAB information, in which the acceptable range of the synchronization is about ±1.5 chip time.
The tracking process controls the timing by a closed loop means similar to that of a Transmit Power Control (TPC) system. That is, the base station transmits the TAB information every 20 ms to the available UEs. The base station sets the TAB information to “0” if the received signal timing is fast and to “1” if the received signal timing is slow. For a frame having 15 slots, the TAB information is included in the 15th slot of every other frame. More particularly, the TAB information is included in the TPC field of the 15th slot. The UE adjusts the timing by a predetermined time unit, which is determined by combining 10 hard decision values of “0” or “1” based on the TAB information. Here, the acceptable range of the synchronization is about ±⅛ chip time, if the tracking process is performed using the TAB information. Also, the timing control interworks with the Dedicated Channel (DCH).
FIG. 1 illustrates a frame structure for a related downlink dedicated channel. The downlink dedicated channel consists of a plurality of radio frames, where a period Tf of one frame is 10 ms. One frame is made of 15 slots (Slot#0˜Slot#14). Each slot time (Tslot) has 2,560 chips and a total number of bits given by 10×2k bits (k=0,1 . . . 7). The slot includes an alternating sequence of a Dedicated Physical Data Channel (DPDCH) and a Dedicated Physical Control Channel (DPCCH). The first DPDCH includes the data1 of Ndata1 bits, and the first DPCCH includes TPC control information of NTPC bits and TFCI control information of NTPCI bits. The second DPDCH includes the data2 of Ndata2 bits, and the second DPCCH includes pilot information of Npilot bits.
The TFCI control information contains the channel information that is currently being transmitted. For example, the TFCI control information may identify the amount of data being transmitted, a coding method, and so on.
The TPC control information originally includes the transmission power control (TPC), but the TAB information may be included in the TPC control information when using the USTS of the present invention. That is, the TPC control information and the TAB information are alternatively included into the TPC field, through the dedicated channel, in the case of the USTS.
A timing renewal period of controlling the timing, based on the TAB information for USTS, is amended from 20 msto 200 ms. Accordingly, the base station transmits ten independent pieces of TAB information, during a 200 ms period, in a manner similar to transmitting one piece of TAB information to the UE during a 20 ms period, as described above. The UE receives the ten pieces of TAB information transmitted from the base station, during the timing renewal period, and determines the timing using this information and adjusts its timing accordingly. This process can be well understood by one of ordinary skill in the art, with reference to the 3GPP Specification.
The continuous transmission of the independent TAB information, during the 200 ms period, impacts the TPC performance due to a puncturing operation, since the TPC bits are reduced. Also, a great deal of time is consumed since the UE combines ten pieces of TAB information to make a timing decision, thereby delaying and decreasing the frequency of the timing adjustments.
Also, the transmission of the independent TAB information itself causes a problem of reliability of the TAB information. The problem will be more serious in a soft hangover area. Accordingly, the data could not be well detected since the USTS gain could not be normally maintained, because of a failure to maintain the timing synchronization of the radio frame.
Further, a method of combining the TAB information is not considered in the related art USTS.
Furthermore, even if the base station transmits the TAB information several times, it does not have a large impact on the decision generated every 200 ms, since the UE performs a timing update based on the TAB information in units of ten.
Meanwhile, if the UE is located in a soft handover area, the TAB information is transmitted from a base station that maintains the USTS. Accordingly, the UE can receive the TAB information at a lower power than a sufficient reference power. As a result, the quality of the TAB information for the USTS cannot be ensured, thereby requiring a suitable solution of combining the TAB information to solve the foregoing problems.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.